Dampers and shock absorbers are known devices which use a hydraulic fluid as the working medium to create damping forces to control or minimize shock and/or vibration. One class of these devices are known as Magnetorheological (MR) fluid devices. MR devices may be of the "rotary-acting" or "linear-acting" variety. Known MR devices include linear dampers, rotary brakes, and rotary clutches. Each MR device employs a Magnetorheological (MR) fluid comprised of soft-magnetic particles dispersed within a liquid carrier. Typical particles include carbonyl iron, and the like, having various shapes, but which are preferably spherical and which exhibit mean dimensions of between about 0.1 .mu.m to about 500 .mu.m. The carrier fluids include low viscosity hydraulic oils, and the like. In operation, these MR fluids exhibit a thickening-like behavior (a rheology change) upon being exposed to a magnetic field. The higher the magnetic flux strength exposed to the fluid, the higher the damping/restraining force or torque that can be achieved within the MR device. Descriptions of prior art MR fluids can be found in WO 94/10694, WO 94/10693, and WO 94/10692 the inventions of which are commonly assigned to the assignee of the present invention.
Prior art MR fluid devices can be found in U.S. application Ser. No. 08/304,005 entitled "Magnetorheological Fluid Devices And Process Of Controlling Force In Exercise Equipment Utilizing Same", U.S. Ser. No. 08/613,704 entitled "Portable Controllable Fluid Rehabilitation Devices", U.S. Ser. No. 08/674,371 entitled" Controllable Brake", U.S. Ser. No. 08/674,179 entitled "Controllable Vibration Apparatus" and U.S. Pat. Nos. 5,547,049, 5,492,312, 5,398,917, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention.
Notably, these MR devices can provide variable control forces or torques, as the case may be, but involve the complexities and cost associated with the required controllers and electromagnetic coils. These complex components may be cost prohibitive in some applications. Therefore, there is a long felt, and unmet, need for a simple and cost effective controllable MR fluid device for providing variability in force and/or torque which varies as a function of position or rotation.